1. Technical Field of the Invention
The present invention relates to ways of obtaining granular, insulating, soft magnetic films of high magnetization and to their possible applications in the microelectronics field and more particularly in radiofrequency (RF) applications.
2. Description of Related Art
Within the context of the invention, the term “granular film” is understood to mean a film formed from two phases, the first being generally dispersed in the second. The first phase here is crystalline and the second is amorphous.
The term “soft magnetic film” is understood to mean a film having a magnetization that is easily reversible, characterized among other things by a low coercive field (Hc≦5 Oe).
The term “high magnetization” is understood to mean a film possessing a high saturation magnetization (Ms>1 T). The term “insulating film” is understood to mean a film having a very low conductivity, i.e., a resistivity ρ≧500 μΩ·cm, and for example a resistivity of ρ≧103 μΩ·cm.
The term “magnetic film for RF applications” is understood to mean a film that satisfies the conventional theory of coherent magnetization rotation described by the celebrated Landau-Lifshitz-Gilbert model on the basis of the existence of an induced uniaxial magnetic anisotropy characterized by an anisotropy field (Hk>Hc).
U.S. Pat. No. 5,573,863 discloses films of soft magnetic alloys comprising a nanocrystalline phase, essentially consisting of cubic Fe, and an amorphous phase comprising a rare earth element or Ti, Zr, Hf, V, Nb, Ta or W and oxygen in a substantial quantity, the two phases being in the form of a mixture. The choice of iron is justified for its high magnetization.
The solution described relies on the known amorphizing character of FeX alloys with X≧15% by weight, where X represents Ti, Zr, Hf, V, Nb, Ta or W, these elements being deposited by sputtering. The suitable magnetic properties are conventionally obtained after partial crystallization of the compound by a magnetothermal treatment at 400° C. after deposition.
This makes it possible to obtain a relatively dense nanocrystalline ferromagnetic phase dispersed in the initial amorphous matrix. By obtaining a microstructure consisting of ferromagnetic grains strongly coupled together on the nanoscale, it is easier to achieve the conditions for obtaining the soft magnetic character of the film.
A first difficulty consists of the fact that the volume fraction of the nanocrystalline (ferromagnetic) phase is generally low (less than 80%), which does not allow a high magnetization to be achieved. Here, the proposed method adds to the known process the reactive aspect, using oxygen. Of course, this leads to oxidation of the film and to an increase in its resistivity. However, the oxidation process is not selective—it relates both to the grains and to the matrix.
The second difficulty consists of the fact that there is an even greater reduction in the magnetization because of the oxidation of the ferromagnetic crystalline phase of Fe. From this it follows that an insulating character cannot be reconciled with high magnetization.
U.S. Pat. No. 5,725,685 discloses soft magnetic alloy films similar to those described in U.S. Pat. No. 5,573,863, with the sole difference that the amorphous phase contains nitrogen in a substantial quantity and not oxygen. This process makes it possible to avoid the problem of oxidation of the ferromagnetic phase and of maintaining a higher magnetization. However, the resistivity levels are markedly too low owing to the absence of oxidation. It also follows from this that it is impossible to reconcile an insulating (or highly resistive) character here with high magnetization.
European Patent Application No. EP 1,361,586 describes a method of producing a thin magnetic film possessing a high magnetization and an insulating character. This film is prepared using the technique of non-reactive cosputtering using two targets composed respectively of a magnetic alloy and of a dielectric. The advantage of this method is that it relies on a non-reactive process (bombardment by only neutral ionic species), preventing the ferromagnetic grains from being oxidized and making it possible in theory to maintain a high magnetization. The method described may either be sequential (alternating deposition of multilayers) or concomitant (simultaneous code position).
The film described is formed from nanometric CoFe ferromagnetic grains (CoFe being chosen for its high magnetization) that are encapsulated in a dielectric matrix, composed, for example, of Al2O3 or SiO2. The difficulty in this case stems from the choice of the CoFe alloy, which is not naturally soft. Thus, the soft magnetic properties of the film can be provided only on condition that the size of the CoFe grains (typically less than 10 nm) are sufficiently reduced and that strong intergranular coupling be maintained, which assumes a relatively small inter-grain distance (typically less than 5 nm).
However, the insulating character requires a certain volume of dielectric material encapsulating the ferromagnetic grains so as to avoid too high a percolation factor. The adjustments in terms of processes (respective volume fractions of the two phases) are, in this sense, contradictory. The use of CoFe alloys, initially justified by a very high intrinsic magnetization, therefore makes this method difficult and limiting. It is therefore impossible to reconcile insulating character with high magnetization.
The trend in the microelectronics field is more and more for ever decreasing individual dimensions of the components in integrated circuits. For certain components this poses a problem.
At the present time, the use of inductors, essentially of planar geometry, within these RF circuits places a limit in terms of the ratio of inductance to area occupied.
Introducing ferromagnetic layers with a high permeability (μ′) allows this ratio to be increased significantly. These layers must meet the constraints of being used at high frequency, especially in dissipative terms, so as to comply with a high quality factor of the component.
Their integration must therefore minimize the additional losses, the origin of which are mainly magnetic (μ″) and capacitive (C). The capacitive losses stem from the juxtaposition of several metal levels separated by dielectrics needed for the fabrication of the component.
The first contribution may be minimized by establishing a high ferromagnetic resonance frequency (FRF) thanks especially to the use of layers with a high saturation magnetization. In certain cases, the aim will on the contrary be to use the adsorbtivity at ferromagnetic resonance (maximum μ″) for electromagnetic screening functionalities. The capacitive contribution remains the more limiting and the more difficult to get round in the current prior art, in which the thin ferromagnetic layers suitable from the magnetic standpoint are conducting in character.
At the present time, known soft magnetic materials with a high magnetization form the FeXN family with X: Al, Si, Ta, Zr, Hf, Rh, or Ti. Unlike U.S. Pat. No. 5,725,685 these materials are obtained directly in the nanocrystallized state with an amorphous matrix by reactive sputtering in a stream of nitrogen.
The incorporation of nitrogen atoms during the growth of the film allows the grain size to be progressively reduced (down to 5 nm) and allows the associated volume fraction to be controlled, which remains high (≧90%). These materials have in general a high magnetization (from 1.8 to 2 T) and excellent soft magnetic properties up to several GHz.
On the other hand, they do not make it possible to achieve optimum results in terms of integration in RF inductive coils (self-inductors). This is because the resistivity (ρ) of these materials remains too low, of the order of 150 μΩ·cm. Despite the dispersion of the conducting FeXN crystalline phase in a resistive amorphous matrix, the overall character of the material remains essentially conducting.
The use of such a conducting material for this type of application is the basis of problems relating to the capacitive coupling between the plane and the inductive coil, which very greatly degrades the load and does not make it possible to obtain quite high quality factor values (typically Q≧30). The FeXO-type materials of insulating character described in the literature themselves do not have suitable magnetic properties (magnetization too low).
The current research remains focused on insulating magnetic materials of high permeability allowing contact between a magnetic plane and an inductive coil, or even the encapsulation of the inductive coil, so as to improve the compactness and the performance of these components in general.
There is a need in the art for a material having the advantage of being both insulating and optimum from the standpoint of the intended magnetic properties. The term “optimum magnetic properties” is understood to mean the combination of a high magnetization (≧1.5 T), a low coercive field (Hc≦5 Oe) and a uniaxial anisotropy field (Hk≧10 Oe). Its insulating property prevents any problem of capacitive coupling between the magnetic plane and the inductive coil, thus making it possible to obtain a maximum gain (≧100%) in the value of the inductive coil and to improve its quality factor. As the insulating film has a very low conductivity, it does not generate supplementary capacitive effect once integrated in an RF device.